The present invention relates to a compensator for liquid crystal display and more particularly to a compensator for a super-twisted nematic (hereinafter referred to simply as "STN") liquid crystal display.
Liquid crystal display occupies an important position in the display field because of such characteristic features as being driven at a low voltage, light weight and low cost. Above all, an STN liquid crystal display is of a multiplex driven dot-matrix type which permits a large-screen display, and as compared with the conventional twisted nematic (TN) type liquid crystal type display the STN liquid crystal display is characteristic in that the contrast is high and the viewing angle is wide. Therefore, the STN liquid crystal display is widely used in the field of liquid crystal display which requires a large-screen display such as, for example, personal computers, word processors and various data terminals.
However, the STN type basically cannot make display in a black-and-white mode and it is unavoidable that the display is in green to yellowish red in the absence of an electric field and in blue in the presence of an electric field. Such a colored mode display is not only undesirable to the users but also involves a serious drawback of being unable to cope with coloring of a display unit. Various proposals have been made for eliminating this drawback. The change from the colored mode to the black-and-white mode can be done basically by making light which has become an elliptically polarized light after passing through a liquid crystal cell again into a linearly polarized light by the use of a compensator. As one method there has been proposed a so-called double STN method wherein the compensation is made by disposing on an STN liquid crystal cell for display another liquid crystal cell for compensation having the same cell gap and a reversed twist angle. Indeed this method permits black-and-white display, but still involves drawbacks; for example, the viewing angle is narrow and coloration is observed when seen from an oblique angle. Additionally, the production of the cell for compensation is difficult, the yield is poor, and the manufacturing cost is very high. These are serious problems. For overcoming such drawbacks of the two-layer cell method it has been proposed to replace the cell for compensation with a single film which exhibits an equivalent optical performance to thereby realize black-and-white display (Japanese Patent Laid Open No.149624/1988). Basically, if it is possible to obtain a uniform film having the same birefringence characteristic, same thickness and same pitch as those of the cell for display and having a reverse twist structure, then black-and-white display can be realized by laminating the said film to the cell for display. Actually, however, the production of such film is extremely difficult, and in the laid-open print referred to above there is found neither description nor working examples at all about the structure and how to produce such film. It can be said that this fact demonstrates the difficulty of embodying the principle in question. As a convenient method for avoiding this difficulty, studies are being made about practical application of films for compensation such as a stretched polycarbonate film adjusted in its birefringence characteristic alone in conformity with the cell for display. However, the compensation effect of these films is insufficient, there merely is obtained a pseudo black-and-white display in bluish white in the absence of voltage, and the contrast is fairly low in comparison with that in the two-layer method. Thus, there is no example of having realized a complete black-and-white display in STN liquid crystal display using film.
It is the object of the present invention to overcome the above-mentioned difficulty involved in films for color compensation in STN liquid crystal display.